[REFACTOR] healthd: BatteryMonitor::update split into 3 funcs

split bool update() into three functions:

void update()
void logValues()
bool isChargerOnline

... so that it doesn't depend on healthd_board_battery_update and
healthd_mode_ops, which are deprecated.

Test: health 2.0 VTS test
Bug: 142260281
Change-Id: I65908a04bae00654213efb4b5569ced0a7e76076